Securities (Narrative) (Details) $ in Thousands	12 Months Ended
Dec. 31, 2020 USD ($) security issuance shares	Dec. 31, 2019 USD ($)
Debt Securities, Available-for-sale [Line Items]
Number Of Issuances Of Securities Exceeding Shareholders Equity Threshold \| issuance	0
Accrued Interest Receivable for securities		$ 46,839
Visa Class B shares, shares sold \| shares	17,360
Visa Class B shares, gain on sale	$ 3,000
Visa Class B shares, remaining shares \| shares	77,683
Equity Securities without Readily Determinable Fair Value, Amount	$ 13,425	0
Other Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Issuances Of Securities Exceeding Shareholders Equity Threshold, Percent	10.00%
Available-for-sale Securities
Debt Securities, Available-for-sale [Line Items]
Accrued Interest Receivable for securities	$ 17,100	$ 13,900
U.S. Government Agency and Government-Sponsored Enterprise Mortgage-Backed Securities and Collateralized Mortgage Obligations
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions \| security	78
Asset-backed Securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions \| security	12
Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Accrued Interest Receivable for securities	$ 54,831
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions \| security	23
US Government Corporations and Agencies Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions \| security	3

Issuances Of Securities Exceeding Shareholders Equity Threshold, Percent

No definition available.

Name:	colb_IssuancesOfSecuritiesExceedingShareholdersEquityThresholdPercent
Namespace Prefix:	colb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Number Of Issuances Of Securities Exceeding Shareholders Equity Threshold

+ References

No definition available.

+ Details

Name:	colb_NumberOfIssuancesOfSecuritiesExceedingShareholdersEquityThreshold
Namespace Prefix:	colb_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Visa Class B shares, gain on sale

+ References

No definition available.

+ Details

Name:	colb_VisaClassBsharesgainonsale
Namespace Prefix:	colb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Visa Class B shares, remaining shares

+ References

No definition available.

+ Details

Name:	colb_VisaClassBsharesremainingshares
Namespace Prefix:	colb_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Visa Class B shares, shares sold

+ References

No definition available.

+ Details

Name:	colb_VisaClassBsharessharessold
Namespace Prefix:	colb_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27290-111563

+ Details

Name:	us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of investment in equity security without readily determinable fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 321
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120269885&loc=SL75117539-209714

+ Details

Name:	us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Carrying amount as of the balance sheet date of interest earned but not received. Also called accrued interest or accrued interest receivable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.8)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_InterestReceivable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FinancialInstrumentAxis=colb_OtherSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AvailableforsaleSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=colb_USGovernmentAgencyAndGovernmentSponsoredEnterpriseMortgageBackedSecuritiesAndCollateralizedMortgageObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MunicipalBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: